1. Field of the Invention
The present invention relates to a surface light source device and, more specifically, to improvement of a surface light source device having a light guide plate for outputting, from its front surface, input light that is input through its incident surface from plural point light sources that are arranged linearly.
2. Description of the Related Art
Surface light source devices are known that illuminate a display panel such as a liquid crystal panel over its entire display area with uniform light. Surface light source devices of this type are classified into a direct type in which a light-emitting element(s) is disposed in the rear of a body and a side light type (also called an edge light type) in which a light-emitting element(s) is disposed in a side portion of a body. Further, side-light-type surface light source devices are classified into a type in which a linear light source such as a CCFL (cold cathode fluorescent lamp) is used as a light-emitting element and a type in which point light sources such as LEDs (light-emitting diodes) are used as light-emitting elements. Side-light-type surface light source devices that employ plural point light sources as light-emitting elements have a problem that luminance unevenness or color unevenness may occur in a region close to the point light sources in their arrangement direction. (See, for example, JP-A-2002-107721.)
FIG. 11 is a sectional view showing the structure of an important part of a conventional surface light source device, and shows a side-light-type surface light source device having a light guide plate 101 that outputs, from its front surface, input light that is input through an incident surface 101a from plural LEDs 102 that are arranged linearly. This surface light source device is composed of a light guide plate 101 and a light source unit that is opposed to one end surface of the light guide plate 101. The light guide plate 101 is a transparent flat plate made of an acrylic resin or glass and has the incident surface 101a on which light coming from the light source unit shines. Light that shines on the incident surface 101a travels through the light guide plate 101 and is output through its front surface. The back surface of the light guide plate 101 is formed with a diffusion pattern 105 for diffusing light traveling through the light guide plate 101. That is, light that has been diffused by the diffusion pattern 105 (i.e., diffused light) is output through its front surface. The light source unit is composed of plural LEDs 102, a board 103 on which the LEDs 102 are formed so as to be arranged linearly, and a reflector 104 for outputting light coming from the LEDs 102 to the light guide plate 101 by reflecting it.
In the above side-light-type surface light source device, the distance between the point light sources (LEDs 102) and the incident surface 101a of the light guide plate 101 cannot be increased beyond a certain limit because it is necessary that what is called a frame area be kept narrow. As a result, light beams emitted from the respective point light sources are not mixed together sufficiently in their arrangement direction when they reach the incident surface 101a. Part of the input light originating from each point light source is directly diffused by a portion C, in the vicinity of the light source unit, of the diffusion pattern 105 and is then output through the front surface of the light guide plate 101. Since light beams coming from adjoining point light sources are output after being diffused in the vicinity of the light source unit without being mixed with each other sufficiently, luminance unevenness (i.e., a bright/dark pattern) may occur in the arrangement direction of the point light sources in a region, in the vicinity of the light source unit, of the front surface of the light guide plate 101. That is, bright portions are seen in regions close to the point light sources and dark portions are seen in regions between the point light sources.
In particular, where LEDs that emit light beams of plural colors (R (red), G (green), and B (blue)) are used as point light source, light beams emitted from the point light sources of the different colors are not mixed together sufficiently and hence color unevenness may occur in addition to luminance unevenness. Luminance unevenness or color unevenness results in a problem that the display quality of a display panel is lowered.
As described above, conventional surface light source devices have the problem that luminance unevenness or color unevenness may occur in a region close to point light sources in their arrangement direction.
The present invention has been made in view of the above circumstances, and an object of the invention is therefore to provide a surface light source device capable of preventing luminance unevenness and color unevenness from occurring in a region close to point light sources in their arrangement direction and thereby increasing the display quality. In particular, it is an object of the invention to provide a surface light source device capable of preventing light beams emitted from adjoining point light sources from being diffused in their vicinities without being mixed with each other sufficiently.